Electrical characterization of interferometric modulators

ABSTRACT

Disclosed herein are methods and systems for testing the electrical characteristics of reflective displays, including interferometric modulator displays. In one embodiment, a controlled voltage is applied to conductive leads in the display and the resulting current is measured. The voltage may be controlled so as to ensure that interferometric modulators do not actuate during the resistance measurements. Also disclosed are methods for conditioning interferometric modulator display by applying a voltage waveform that causes actuation of interferometric modulators in the display.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/743,594, filed May 2, 2007, which is a divisional of U.S. patentapplication Ser. No. 11/097,511, filed Apr. 1, 2005, now U.S. Pat. No.7,289,256, which claims the benefit of U.S. Provisional Application No.60/613,537, filed on Sep. 27, 2004. This application is also related toU.S. patent application Ser. No. 12/041,618, filed Mar. 3, 2008, nowU.S. Pat. No. 7,580,176. The above-reference applications and patentsare hereby incorporated by reference herein in their entirety.

BACKGROUND

1. Field of the Invention

The field of the invention relates to microelectromechanical systems(MEMS).

2. Description of the Related Technology

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. An interferometricmodulator may comprise a pair of conductive plates, one or both of whichmay be transparent and/or reflective in whole or part and capable ofrelative motion upon application of an appropriate electrical signal.One plate may comprise a stationary layer deposited on a substrate, theother plate may comprise a metallic membrane separated from thestationary layer by an air gap. Such devices have a wide range ofapplications, and it would be beneficial in the art to utilize and/ormodify the characteristics of these types of devices so that theirfeatures can be exploited in improving existing products and creatingnew products that have not yet been developed. In order to ensure highquality, accurate and convenient methods for testing the operation ofsuch MEMS devices may be employed in the manufacturing process. Furtherdevelopment of such methods is needed.

SUMMARY OF CERTAIN EMBODIMENTS

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Certain Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

One embodiment includes a method of measuring resistance betweenconductive leads in a display, comprising applying a controlled voltageacross at least two conductive leads in the display, wherein theconductive leads are used for driving display elements within thedisplay and measuring current through the leads in response to theapplied voltage.

Another embodiment includes a method of testing an interferometricmodulator display, comprising applying a controlled voltage across atleast two conductive leads in the display, wherein the conductive leadsare used for driving interferometric modulators within the display;measuring current through the leads in response to the voltage; andidentifying the display as having electrical characteristics desirablefor use as a display based on the measuring.

Another embodiment includes a method of conditioning an interferometricmodulator display, comprising applying, prior to use of the display, avoltage waveform to the display, wherein the voltage waveform has anamplitude high enough to actuate at least one interferometric modulatorin the display, wherein the voltage waveform is such that it supplies anet zero charge to the interferometric modulators in the display.

Another embodiment includes a method of repairing a short in a display,comprising applying a voltage across at least two conductive leadsthrough which a short has been measured, the voltage sufficient toactuate at least one display element through which the conductive leadspass.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a released position and amovable reflective layer of a second interferometric modulator is in anactuated position.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIG. 6A is a cross section of the device of FIG. 1.

FIG. 6B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 6C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7 is a flowchart illustrating a method of measuring the electricalcharacteristics of a display such as an interferometric modulatordisplay.

FIG. 8 is an illustration of an interferometric modulator array withbuss bars, test pads, and shorting bars.

FIG. 9 is another illustration of an interferometric modulator arraywith buss bars, test pads, and shorting bars.

FIG. 10A is a graph illustrating an alternating square voltage waveformfor conditioning an interferometric modulator array.

FIG. 10B is a graph illustrating a triangular voltage waveform forconditioning an interferometric modulator array.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theinvention may be implemented in any device that is configured to displayan image, whether in motion (e.g., video) or stationary (e.g., stillimage), and whether textual or pictorial. More particularly, it iscontemplated that the invention may be implemented in or associated witha variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

After manufacture of an interferometric modulator display, it may bedesirable to test the display for certain operational characteristics todetermine whether the display is suitable for use or to diagnose anyinherent manufacturing defects. Characteristics desirable to testinclude electrical characteristics, such as the resistance of theconductive leads within the display. Measurement of opens throughconductive leads indicates a fault with that lead. Measurement of shortsbetween adjacent conductive leads indicates that the leads arecontacting each other or that conductive debris is present between theleads. Thus, disclosed herein are methods and systems for testing theelectrical characteristics of interferometric modulator displays.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as thereleased state, the movable layer is positioned at a relatively largedistance from a fixed partially reflective layer. In the secondposition, the movable layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable and highly reflective layer 14 ais illustrated in a released position at a predetermined distance from afixed partially reflective layer 16 a. In the interferometric modulator12 b on the right, the movable highly reflective layer 14 b isillustrated in an actuated position adjacent to the fixed partiallyreflective layer 16 b.

The fixed layers 16 a, 16 b are electrically conductive, partiallytransparent and partially reflective, and may be fabricated, forexample, by depositing one or more layers each of chromium andindium-tin-oxide onto a transparent substrate 20. The layers arepatterned into parallel strips, and may form row electrodes in a displaydevice as described further below. The movable layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes 16 a, 16 b) deposited on top ofposts 18 and an intervening sacrificial material deposited between theposts 18. When the sacrificial material is etched away, the deformablemetal layers are separated from the fixed metal layers by a defined airgap 19. A highly conductive and reflective material such as aluminum maybe used for the deformable layers, and these strips may form columnelectrodes in a display device.

With no applied voltage, the cavity 19 remains between the layers 14 a,16 a and the deformable layer is in a mechanically relaxed state asillustrated by the pixel 12 a in FIG. 1. However, when a potentialdifference is applied to a selected row and column, the capacitor formedat the intersection of the row and column electrodes at thecorresponding pixel becomes charged, and electrostatic forces pull theelectrodes together. If the voltage is high enough, the movable layer isdeformed and is forced against the fixed layer (a dielectric materialwhich is not illustrated in this Figure may be deposited on the fixedlayer to prevent shorting and control the separation distance) asillustrated by the pixel 12 b on the right in FIG. 1. The behavior isthe same regardless of the polarity of the applied potential difference.In this way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application. FIG. 2is a system block diagram illustrating one embodiment of an electronicdevice that may incorporate aspects of the invention. In the exemplaryembodiment, the electronic device includes a processor 21 which may beany general purpose single- or multi-chip microprocessor such as an ARM,Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051,a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessorsuch as a digital signal processor, microcontroller, or a programmablegate array. As is conventional in the art, the processor 21 may beconfigured to execute one or more software modules. In addition toexecuting an operating system, the processor may be configured toexecute one or more software applications, including a web browser, atelephone application, an email program, or any other softwareapplication.

In one embodiment, the processor 21 is also configured to communicatewith an array controller 22. In one embodiment, the array controller 22includes a row driver circuit 24 and a column driver circuit 26 thatprovide signals to a pixel array 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the released state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not releasecompletely until the voltage drops below 2 volts. There is thus a rangeof voltage, about 3 to 7 V in the example illustrated in FIG. 3, wherethere exists a window of applied voltage within which the device isstable in either the released or actuated state. This is referred toherein as the “hysteresis window” or “stability window.” For a displayarray having the hysteresis characteristics of FIG. 3, the row/columnactuation protocol can be designed such that during row strobing, pixelsin the strobed row that are to be actuated are exposed to a voltagedifference of about 10 volts, and pixels that are to be released areexposed to a voltage difference of close to zero volts. After thestrobe, the pixels are exposed to a steady state voltage difference ofabout 5 volts such that they remain in whatever state the row strobe putthem in. After being written, each pixel sees a potential differencewithin the “stability window” of 3-7 volts in this example. This featuremakes the pixel design illustrated in FIG. 1 stable under the sameapplied voltage conditions in either an actuated or releasedpre-existing state. Since each pixel of the interferometric modulator,whether in the actuated or released state, is essentially a capacitorformed by the fixed and moving reflective layers, this stable state canbe held at a voltage within the hysteresis window with almost no powerdissipation. Essentially no current flows into the pixel if the appliedpotential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Releasing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias). As is also illustrated in FIG.4, it will be appreciated that voltages of opposite polarity than thosedescribed above can be used, e.g., actuating a pixel can involve settingthe appropriate column to +V_(bias), and the appropriate row to −ΔV. Inthis embodiment, releasing the pixel is accomplished by setting theappropriate column to −V_(bias), and the appropriate row to the same−ΔV, producing a zero volt potential difference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or released states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and releases the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and release pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thepresent invention.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 6A-6C illustrate three different embodiments of themoving mirror structure. FIG. 6A is a cross section of the embodiment ofFIG. 1, where a strip of metal material 14 is deposited on orthogonallyextending supports 18. In FIG. 6B, the moveable reflective material 14is attached to supports at the corners only, on tethers 32. In FIG. 6C,the moveable reflective material 14 is suspended from a deformable layer34. This embodiment has benefits because the structural design andmaterials used for the reflective material 14 can be optimized withrespect to the optical properties, and the structural design andmaterials used for the deformable layer 34 can be optimized with respectto desired mechanical properties. The production of various types ofinterferometric devices is described in a variety of publisheddocuments, including, for example, U.S. Published Application2004/0051929. A wide variety of well known techniques may be used toproduce the above described structures involving a series of materialdeposition, patterning, and etching steps.

Electrical Characterization

In some embodiments, methods are provided for testing the electricalcharacteristics of an interferometric modulator display. In some cases,the resistance across conductive leads in the display is measured.Resistance measurements can indicate whether the conductive leads wereproperly formed during the manufacturing process and whether any debriswithin the display is altering the electrical characteristics. Forexample, it is desirable that the resistance through each individualconductive lead (e.g., each row and column conductive lead) be low. Ahigh resistance through an individual conductive lead may be indicativeof an open within the lead. It is also desirable that the resistancebetween adjacent conductive leads be high. For example, a low resistancebetween a column and a row conductive lead may indicate that a shortexists in the interferometric modulator element that is formed at theintersection of the column and row. Similarly, a low resistance betweenadjacent column or row conductive leads may be indicative of a shortbetween those leads. Shorts may be caused by an error in manufacturingthat causes the conductive material to be fused together or byconductive debris between the leads.

Measuring resistance in conductive leads in an interferometric modulatordisplay includes determining the resistance in view of the fact that theelectrical characteristics of the display can vary depending on whetherthe interferometric modulator elements are actuated or not. In typicalresistance meters, such as those typically used to test liquid crystaldisplays, the voltage applied to make the measurement may vary.Accordingly, the voltage may rise to levels sufficient to actuateinterferometric modulators within the display, which may result indifferent measurements from those obtained when the interferometricmodulators do not actuate. Thus, in one embodiment, resistance ismeasured by applying a controlled voltage across the leads and thenmeasuring the resulting current, known as a force voltage/measurecurrent (FVMI) technique, when the interferometric modulators are in anon-actuated state. By applying a controlled voltage, actuation of theinterferometric modulators can be controlled. In one embodiment, suchresistance measurements may be made using a Keithly 6517 high resistancemeter.

FIG. 7 is a flowchart illustrating a method of measuring resistancebetween conductive leads in a display such as an interferometricmodulator display. Depending on the particular embodiment, steps may beadded to those depicted in FIG. 7 or some steps may be removed. Inaddition, the order of steps may be rearranged depending on theapplication. At step 200, a controlled voltage is applied across atleast two conductive leads of the display. In one embodiment, multipleleads are interfaced to external electronics and the leads to which thecontrolled voltage is applied is determined by the external electronics.For example, all leads in the display could be coupled to externalelectronics containing relays, which could be used for selectivelyapplying a controlled voltage to less than all leads in the display. Insome embodiments, the external electronics can be used to short one ormore leads together. In one embodiment when an interferometric modulatoris being tested, the applied controlled voltage is kept low enough suchthat no interferometric modulators actuate. For example, the voltage maybe kept within +/−1 V. Next, at step 202, the resulting current ismeasured through the conductive leads, after which resistance may bedetermined. In one embodiment, resistance is measured through one ormore conductive leads, such as through a row or column, by applying avoltage to opposite ends of the rows or columns. Such a measurementallows determination of whether an open exists in the rows or columns.In another embodiment, resistance is measured between a row and acolumn. Such a measurement allows determination of whether a shortexists between the row and column in the interferometric modulatorelement at the intersection of the row or column. In one embodiment, aresistance of less than 100 MΩ indicates that unacceptable conductionexists between a row and column. In another embodiment, a resistance ofless than 50 MΩ indicates that unacceptable conduction exists between arow and column. In another embodiment, resistance is measured betweenadjacent rows and/or columns. Such a measurement allows determination ofwhether a short exists between the adjacent rows or columns. In oneembodiment, a resistance of less than 10 MΩ indicates that unacceptableconduction exists between adjacent rows or columns. In anotherembodiment, a resistance of less than 1 MΩ indicates an unacceptableconduction between adjacent rows or columns.

In some embodiments, resistance measurements may be made on multipleconductive leads simultaneously. For example, resistance may be measuredbetween all adjacent rows simultaneously. If the per lead resistancemeasured in this way is less than a predefined threshold, thensubsequent testing may optionally be used to identify which row-rowleads contain a short. Similarly, the resistance of all adjacent columnleads may be measured simultaneously or the row-to-column resistancethrough pixel elements may be measured simultaneously. In someembodiments, the display being tested is a color display. In such cases,the resistance between adjacent leads may include measuring resistancebetween leads for driving different color subpixels, such as between alead for red subpixels and a lead for green and/or blue subpixels.Accordingly, in some embodiments, measuring resistance between adjacentcolumn leads may involve three separate measurements—the red-blue leads,blue-green leads, and red-green leads.

In some embodiments, the controlled voltage is a time-varying voltagewaveform. For example, in some embodiments, an AC waveform is applied. Atime-varying voltage waveform that is symmetric about some constantvalue may be used to ensure that a net zero charge is supplied todisplay elements. For example, when applying the voltage to a row andcolumn to measure for shorts through a display element at the row-columnintersection, a voltage waveform symmetric about 0 V or an offsetvoltage (e.g., the voltage required for zero charge in a displayelement) may be applied to ensure no build up of charge in the displayelement during testing.

In one embodiment, resistance is measured separately for each polarity.The comparison of resistances measured for opposite polarities mayprovide a check of quality of the display. For example, in aninterferometric modulator display, the driving schemes may involveapplying voltages of both polarities as described above. Accordingly, itmay be desirable for the electrical characteristics of the display to besimilar for both polarities. Furthermore, measuring resistance in bothpolarities may provide a check to determine whether detected values aredue to noise or due to actual resistance. For example, if the resistanceis very high, only noise may be measured. If the polarity of themeasured current does not change sign upon reversal of the polarity ofthe applied voltage, there may be indication that only noise is beingdetected.

The resistance measurement may optionally be used to identify thedisplay as having the electrical characteristics desirable for use as adisplay. Thus, for example, proceeding to step 204 in FIG. 7, theresistance measurements or number of shorts and/or opens are compared topredefined thresholds. If the measurements or numbers are within thethresholds, then the display may be used as a display by proceeding tostep 206. If the measurements or numbers are not within the thresholds,then the display may be deemed defective and discarded at step 208. Suchtesting may be used for quality control purposes during manufacture ofthe displays. In some embodiments, a sampling of displays producedduring manufacturing is tested to represent the quality of one or morelots of displays.

In other embodiments, resistance measurements may be used to identifyerrors in manufacturing. For example, resistance measurements may beused to pinpoint locations of manufacturing error. If a pinpointedlocation of error, such as a short through a single pixel, is repeatedlymeasured at the same location in different displays, there may be anindication of a manufacturing error. This information may be used tocorrect defects in equipment or procedure.

In some embodiments, resistance measurements may be used to monitor theresults of certain manufacturing processes. For example, resistancemeasurements may be used to monitor variation in film parameters (e.g.,thickness and width dimensions).

In some embodiments, the precise location of a short or open may bedetermined through a decision tree approach. For example, all leads maybe measured at a first level of the decision tree. If the measuredresistance indicates that a short exists somewhere in the display, halfof the row or column leads may then be tested followed by the other halfif the short or open was not detected in the first half. In this way,the location of the fault is narrowed down to half of the display. Thisprocess may continue in a similar fashion to narrow down the location ofthe fault until it is precisely identified through a single row and/orcolumn. In other embodiments, each row and/or column combination may beseparately measured to determine the location of faults.

In still other embodiments, bypass structures may be incorporated withina display that enables one or more pixels to be bypassed. Thus, forexample, if a resistance measurement indicates a short at one pixelsite, that pixel may be bypassed by activating a bypass structure thatshunts the conductive row and column leads at the pixel around the pixelsite, thus electrically isolating the bad pixel. Accordingly, in oneembodiment, resistance measurement procedures such as described hereinare used to provide an indication of which pixels or regions of adisplay to bypass with an appropriate bypass structure.

Electrical Measurement Structures

In some embodiments, appropriate conductive structures are interfaced toan interferometric modulator array during manufacturing in order tofacilitate electrical and electro-optical characterization and testingof the array. In one embodiment, depicted in FIG. 8, row and columnleads 400 may be connected to one or more buss bars 410, which are thenconnected to one or more test pads 420. Row and column leads 400 maycorrespond, for example, to the row and column strips depicted in FIG.5A. The buss bars 410 electrically connect all the leads connected tothe bar and thus facilitate the voltage control of all those leadssimultaneously. It may also be desirable that all leads are shortedtogether during processing by using a shorting bar 430. The shorting bar430 may be removed prior to electrical and/or electro-optical testing.In one embodiment, alternating row and column leads 400 are connected tothe same buss bars 410 (i.e., interdigitated), as depicted in FIG. 8, toallow for measurement of adjacent row-row or column-column resistancesas described above. As used herein, a “row” refers to the conductinglines adjacent to the substrate (e.g., lines of transparent conductor).As used herein, a “column” refers to the conducting lines associatedwith the movable mirrors. In other embodiments, alternative arrangementsof shorting bars and test pads may be used. For example, the arrangementin FIG. 9 provides test pads 510 on opposite ends of the same row andcolumn (e.g., test pads L1-L2, L3-L4, L7-L8, and L5-L6). Arrangements ofbuss bars 520 and test pads 530 may also be used to test differentregions of the display separately. When an interferometric modulatordisplay is a color display, buss bars and test pads may be constructedso that the different color subpixels (i.e., interferometric modulatorsdesigned to reflect a certain color) can be driven separately. Aftertesting, the buss bars and test pads may be removed so that the activearea (interferometric modulator array) can be incorporated into thedesired user package. In an alternative embodiment, probes are contactedto each lead separately rather than through test pads. In thisembodiment, external electronics may be used to short multiple leadstogether in order to simultaneously measure resistance through multipleleads. Those of skill in the art will recognize many suitable structuresand techniques that can be used to interface an interferometricmodulator array to a resistance measuring device.

In one embodiment, structures such as described in FIGS. 8 and 9 areused to measure resistance across single row and/or column lines (e.g.,across L1-L2, L3-L4, L7-L8, and L5-L6) individually. A resistance thatis too large may indicate a break in the row or column line and thusthat at least a portion of the row or column cannot be matrix addressed.In one embodiment, structures such as described in FIGS. 8 and 9 areused to measure row-to-row and/or column-to-column resistance. If therows and/or columns are interdigitated, then resistance measurementbetween adjacent rows and/or columns will provide a measurement ofcurrent leakage (e.g., measured across R1-R2 or C1-C2 in FIG. 8). Insome embodiments, if total row-row or column-column resistance is toolow, individual sets of adjacent row-row or column-column resistance maybe measured. In one embodiment, row-to-column resistance is measured.This measurement may be made by measuring resistance between all rowsshorted and all columns shorted (e.g., between R1 and R2 shorted and C1and C2 shorted in FIG. 8). Resistance that is too low may indicate ashort between a row and a column.

Interferometric Modulator Conditioning

In one embodiment, performance of an interferometric modulator displayafter manufacture may be improved by preconditioning the display. Thepreconditioning may be accomplished by applying a voltage to the displaysufficient to actuate interferometric modulator elements in the display.Immediately after manufacture, the voltages at which interferometricmodulators actuate may vary until a steady state behavior is reached.Thus, preconditioning may stress the movable interferometric modulatorelements so that a stable or near stable response is achieved uponactuation. Furthermore, such preconditioning may remove transitoryshorts between conductive leads by vaporizing conductive debris. In someembodiments, preconditioning may reveal defects not observed prior topreconditioning. Thus, for example, resistance measurements such asdescribed above may be conducted both before and after preconditioning.

In one embodiment, the preconditioning voltage waveform is applied tosubstantially all elements in the interferometric modulator displaysimultaneously. In such a manner, each element may be stressed andconditioned identically so that the display response of each element issimilar, reducing the observance of ghosting effects.

In one embodiment, a voltage waveform is applied to the display havingan amplitude sufficient to actuate the interferometric modulators. Thevoltage may be applied to all interferometric modulators simultaneously(e.g., by applying the voltage between R1 and R2 shorted to ground andC1 and C2 shorted to a waveform generator in FIG. 8) or to a subset ofthe interferometric modulators. In one embodiment, a voltage waveformthat is symmetric about some constant value may be used to ensure that anet zero charge is supplied to the display elements. For example, avoltage waveform symmetric about 0 V or an offset voltage (e.g., thevoltage required for zero charge in a display element) may be applied toensure no build up of charge in the display elements.

In one embodiment, the voltage waveform includes pulsing of analternating square waveform. FIG. 10A illustrates one such possiblevoltage waveform. A series of square waves having amplitudes 600sufficient to actuate the interferometric modulators may be applied.Thus, when the voltage is at the positive 600 or negative 602 amplitudevalues, the interferometric modulators are actuated. When the voltage isat the offset potential 604, the interferometric modulators are in anon-actuated state. Each square waveform may have width 606 (e.g., 5 ms)before the polarity of the applied voltage is reversed. A series of suchalternating square waveforms may have width 608 (e.g., 0.5 s). Afterapplying this sequence, the voltage may be held at the offset potential604 for time 610 (e.g., 0.5 s). Thus, the result of the waveform in FIG.10A is that interferometric modulators will cycle through the sequenceof an actuated state for time 608 followed by a non-actuated state fortime 610. By making the waveform symmetric about the offset voltage 604and quickly varying the amplitude between positive and negativepolarities when driving the interferometric modulators in an actuatedstate, no net charge is accumulated in the interferometric modulators.Those of skill in the art will recognize many variations of thiswaveform. For example, time periods 606, 608, and 610 may be varied toobtain different frequency of actuation pulsing (e.g., by varying times608 and/or 610) and polarity pulsing (e.g., by varying time 606). Invarious embodiments, the actuation frequency may be at least about 0.1Hz., 0.5 Hz, 1 Hz, 10 Hz, 50 Hz, 100 Hz, 500 Hz, or 1 kHz. In variousembodiments, the polarity change frequency may be at least about 100 Hz,1 kHz, 5 kHz, 10 kHz, 50 kHz, 100 kHz, 500 kHz, and 1 MHz. Furthermore,in some embodiments, a single actuation waveform having alternatingpolarity is applied (e.g., only period 608).

In another embodiment, the voltage waveform includes a triangularwaveform. FIG. 10B illustrates one such possible waveform. Theamplitudes 650 of the triangular waveform are high enough such that theinterferometric modulators actuate before the amplitudes are reached. Inone embodiment, amplitudes that are about 10% higher than the requiredactuation voltage are used. The interferometric modulators willde-actuate before the voltage reaches the offset voltage 652 about whichthe interferometric modulators are centered. In various embodiments, thefrequency of the triangle waveform may be at least about 0.1 Hz., 0.5Hz, 1 Hz, 10 Hz, 50 Hz, 100 Hz, 500 Hz, or 1 kHz.

Those of skill in the art will recognize many possible actuation voltagewaveforms that may be used to precondition interferometric modulators.Thus, the disclosure is not limited to only square and triangularwaveforms having the characteristics described above.

In some embodiments, different waveforms are combined in series tocreate a more complex waveform string. For example, the triangle andsquare waveforms described above may be combined in series. In oneembodiment, the triangle waveform is applied for a first time period(e.g., about 1 minute) followed by multiple sequences of squarewaveforms (e.g., each about 1 minute with increasing amplitudes)followed by a second triangle waveform. This sequence may be repeatedany number of times or varied to produce any number of waveformcombinations. Those of skill in the art will recognize many othervoltage waveforms and combinations of waveforms that may be applied toresult in conditioning of the interferometric modulator elements in adisplay.

In various embodiments, variations in preconditioning voltage waveformsinclude varying the length of time a particular waveform is applied,varying the frequency of the waveform, and varying the amplitude of thewaveform.

In some embodiments, if electrical characterization such as describedabove indicates that an interferometric modulator display isunsatisfactory, conditioning may be employed to improve thecharacteristics. For example, conditioning may remove shorts measuredduring electrical characterization. Thus, in one embodiment, electricalcharacterization is conducted both before and after conditioning todetermine if any faulty characteristics have been corrected byconditioning.

Although the invention has been described with reference to embodimentsand examples, it should be understood that numerous and variousmodifications can be made without departing from the spirit of theinvention. Accordingly, the invention is limited only by the followingclaims.

1. A method of conditioning a microelectromechanical device, the methodcomprising: applying a conditioning signal to a microelectromechanicaldevice having an offset voltage of a first value, the conditioningsignal having an average of a second value, wherein the second value isbased, at least in part, on the first value.
 2. The method of claim 1,wherein the second value is the same as the first value.
 3. The methodof claim 1, wherein the second value is non-zero.
 4. The method of claim1, wherein the conditioning signal is applied prior to the use of themicroelectromechanical device.
 5. The method of claim 1, wherein thevoltage waveform includes an alternating square waveform.
 6. A method ofconditioning a microelectromechanical device, the method comprising:determining an offset value based upon a first conditioning signal to atleast one microelectromechanical device; and applying a secondconditioning signal to a second microelectromechanical device, theaverage value of the second conditioning signal being based, at least inpart, by the determined offset value.
 7. The method of claim 6, whereinthe average value of the second conditioning signal is the same as theoffset value.
 8. The method of claim 6, wherein the average value of thesecond conditioning signal is non-zero.
 9. The method of claim 6,wherein the conditioning signal is applied prior to the use of themicroelectromechanical device.
 10. The method of claim 6, wherein thevoltage waveform includes an alternating square waveform.
 11. A methodof conditioning a microelectromechanical device, the method comprising:applying a conditioning signal having a non-zero average to amicroelectromechanical device such that the resultant offset voltage isless than would result from the same waveform with a zero average.